My boss thinks McGregor will be the winner. I'm a lot less certain. Sure Mayweather is older and that can work against him, I think he has a few things going for him.
First, he's a boxer. He specializes in boxing whereas McGregor as an MMA fighter will be under boxing rules and he may have techniques that usually work for him but he won't be able to use.
The second is experience. I think Mayweather has a lot more of it and that all things be equal, that would put him at an advantage.